1. Field of the Invention
The invention relates to a pressure limiting valve.
2. Description of the Prior Art
Pressure limiting valves are required for example in gas filling stations in which mobile pressure containers, such as e.g. the supply container of a gas-operated motor vehicle, are filled with gas. Gas filling stations of this kind typically comprise a stationary storage container which is filled with compressed gas and a discharge device in order to connect this storage container to the mobile supply container, so that the gas can flow from the storage container into the mobile supply container.
Compressed natural gas above all is increasingly gaining in importance as an alternative fuel for motor vehicles. In order to enable a satisfactory range of motor vehicles which are operated with natural gas and at the same time to keep the dimensions of the supply container in the motor vehicle within reasonable limits, these supply containers are typically filled up to pressures of approximately 200 bar with respect to a reference temperature of 15xc2x0 C. For this, filling methods and stations have been developed which enable a very simple and rapid filling of motor vehicles of this kindxe2x80x94in comparison with the filling with gasoline. A method of this kind and a gas filling station of this kind respectively are described in detail in EP-A-653 585.
A problem in gas filling stations consists in that the ambient temperature which is present during the filling must be taken into account for the final pressure up to which the mobile supply container is to be filled. If it is namely assumed that this final pressure should amount to approximately 200 bar at a reference temperature of 15xc2x0 C., then it is clear that at an outer temperature of less than 15xc2x0 C. the final pressure at which the filling is terminated must amount to less than 200 bar in order to ensure that no impermissibly high pressure arises in the mobile supply container in the event that the ambient temperature rises. Conversely, the filling can be carried out up to a final pressure of more than 200 bar by an ambient temperature of more than 15xc2x0 C. without the danger that a too high pressure in the supply container arises.
In fact, it is possible to measure the fluctuations of the ambient temperature via temperature sensors and then to realize the correct, temperature corrected final pressure for the filling by means of suitable control devices, but methods of this kind are however relatively complex and expensive. It is therefore desirable to have a pressure limiting valve available which takes temperature fluctuations of this kind into account without assistance.
The object of the invention is thus to provide as simple a pressure limiting valve as possible which automatically varies the limiting pressure at which it opens or closes respectively in dependence on temperature.
In accordance with the present invention a pressure limiting valve is proposed which comprises an inlet and an outlet for a fluid, which comprises a valve body which cooperates with a valve seat in such a manner that it opens or closes a flow connection for the fluid between the inlet and the outlet when a limiting pressure is reached, and which comprises a spring element which acts on the valve body and loads the latter, with a container for a liquid furthermore being provided which is arranged and designed in such a manner that the liquid varies through its thermal expansion the loading of the valve body which is caused by the spring element and thus varies the limiting pressure in dependence on the temperature of the liquid.
The volume of the liquid in the container varies as a result of its thermal expansion. If for example the temperature of the liquid rises, then its volume increases. Through this increase in volume the spring element is compressed, whereby the loading of the valve body which is caused by the spring element rises. Thus the limiting pressure also rises at which the pressure limiting valve closes or opens respectively. Conversely, the volume of the liquid decreases when the temperature decreases. Thereby, the spring element is somewhat relaxed and the loading of the valve body decreases. As a result the limiting pressure at which the valve closes or opens drops. Thus the pressure limiting valve automatically varies its limiting pressure in dependence on the temperature, whereby a temperature dependent pressure limitation is enabled in a simple manner.
The container for the liquid is preferably designed as a hollow cylinder and comprises a movable pressure piston, which is braced at the one side on the spring element and which is charged at the other side by the pressure of the liquid.
In a first preferred embodiment the inner wall of the hollow cylinder guides the pressure piston, which means that the inner diameter of the hollow cylinder is substantially the same as the diameter of the pressure piston. The movable pressure piston forms a boundary surface of the volume which is available to the liquid in the interior of the hollow cylinder.
In a second preferred, particularly compact embodiment an inner cylinder is provided which is arranged coaxially in the hollow cylinder, with the pressure piston being provided in the inner cylinder and being guided by the latter, and with the inner cylinder having an opening so that the liquid can act on the pressure piston.
The cross-sectional area of the pressure piston is preferably larger than the effective cross-sectional area of the valve body which is charged by the pressure of the fluid. This measure namely has the advantage that the pressure of the liquid in the container can be chosen significantly less than the limiting pressure at which the valve closes or opens respectively.
In accordance with a further development of the pressure limiting valve in accordance with the invention, means are provided for a thermal contact between the fluid and the liquid so that the liquid in the container has substantially the same temperature as the fluid. For this for example a pressure line can be provided in the container which is surrounded by the liquid and through which the fluid flows. In accordance with the principle of the heat exchanger the liquid then assumes the same temperature as the fluid. As a result the liquid varies the limiting pressure of the pressure limiting valve in dependence on the current temperature of the fluid.
The container preferably comprises setting means in order to vary the volume which is available to the liquid. These setting means serve for the adjusting or the calibrating respectively of the pressure limiting valve. The setting means comprise for example an adjusting piston which is matched to the inner diameter of the hollow cylindrical container and the position of which can be varied via a setting screw. By rotating the setting screw the adjusting piston can be displaced along the longitudinal axis of the hollow cylinder, whereby the volume which is available to the liquid can be varied. For adjusting the pressure limiting valve the position of the adjusting piston and thus the tension of the spring element is varied at a known reference temperature until the limiting pressure belonging to this reference temperature is reached.
In order to achieve a sufficiently large temperature dependence of the limiting pressure of the pressure limiting valve, the container preferably contains a liquid in the operating state, the thermal coefficient of volume expansion of which amounts to at least 10xe2x88x924 Kxe2x88x921, in particular to at least 5xc2x710xe2x88x924 Kxe2x88x921. It is particularly preferred to use oil, which typically has coefficients of volume expansion of this magnitude, as the liquid.
In particular in regard to gas filling stations, especially for natural gas, in which the pressure limiting valve serves to close the pressure line when the temperature dependent filling pressure has been reached, the limiting pressure is preferably linearly dependent on the temperature of the liquid, with the slope amounting in particular to from 1.5 bar/K to 2 bar/K, because this slope corresponds to the pressure-temperature relationship of natural gas. The slope of the limiting pressure as a function of the temperature can be set to the desired value in a simple manner by the amount of liquid in the container in the pressure limiting valve in accordance with the invention.
In gas filling stations for natural gas the pressure limiting valve is preferably set in such a manner that the limiting pressure amounts to from 180 bar to 220 bar when the liquid in the container has reached a temperature of 15xc2x0 C.